Add item to basket

Adds a listing to the basket and gives you the updated basket state back.

Typical usage:

• user clicks "Add to basket" on a listing page
• client sends listingId and quantity
• UI refreshes from the returned basket payload

Why this response is handy:

• You can re-render the whole basket immediately without an extra read call.

Important behavior:

• Requires authentication.
• If stock is unavailable (or the listing is invalid), the API returns structured listing errors.